The present invention relates to a distributor for a hydraulic hoist which is controllable with regard to position and applied force, for tractors and agricultural machines. In particular, this distributor is to be used to regulate and control the movement of a three-point coupling which is mounted on the tractor or agricultural machine and to which an implement for working or preparing the ground is connected.
Various types of hoist for three-point couplings and various types of distributor which are to control the movement thereof are known. One of those known distributors is described, for example, in Italian patent application MO91A000137 belonging to the same applicant. Basically, distributor of the known type comprises a substantially cylindrical and compact body in which various valve elements are arranged, the most important of which is a slide valve of which the slider is operated by the hoist levers and which permits modulation of a specific rate of flow of pressurised fluid to an actuator cylinder or, alternatively, discharge from that actuator cylinder of a specific amount of fluid. Hoists are normally connected to the pressurised oil distribution system which is already arranged on the agricultural machine for other equipment.
Oil distribution systems normally comprise positive-displacement pumps which, in most tractors, are of the fixed displacement type because they are far less expensive than the variable displacement type. It often happens that the oil flow necessary for the ancillary equipment of the tractor is very high; in that case, above all in the presence of fixed displacement positive-displacement pumps and of medium-low displacements of the hoist cylinders, distributors of the known type may exhibit an oscillatory behaviour owing to an instability caused by the system's excessive rate of response. In addition, the distributor is subject to pressure peaks which may prove to be detrimental both to the strength and to the precision of the system.
In order to solve the above problems the known solutions necessarily involve a substantial increase in the dimensions of the distributor or a complication of the system with the addition of separate valves in positions to which access is difficult.